1. Field of the Invention
The present invention relates to an image pickup apparatus such as a digital camera or a video camera, and more particularly to an image pickup apparatus which performs focus detection based on an output from an image pickup element.
2. Description of the Related Art
Japanese Patent No. 3592147 discloses an image pickup apparatus which performs focus detection using an image pickup element. The image pickup element of the image pickup apparatus includes a plurality of pairs of focus detection pixels which have optical characteristics different from those of other pixels in a part of pixels constituting the image pickup element, and performs the focus detection based on outputs of the focus detection pixels. In the image pickup apparatus, an image signal corresponding to a signal from the focus detection pixel is interpolated using an image signal of its peripheral pixel.
Japanese Examined Patent Publication No. 7-62731 discloses an automatic focus detection apparatus which detects the vignetting of an incident light beam to switch an aperture of an optical system. The automatic focus detection apparatus includes a first focus detector which receives a light beam having a first aperture value with respect to a pair of light beams passing through optical systems, which are different from each other, of an image pickup optical system from an object, and a second focus detector which receives a light beam having a second aperture value greater than the first aperture value. Further, the automatic focus detection apparatus includes an unbalance state detector which detects the vignetting in the light beam having the first aperture value, and is configured to switch between the first focus detector and the second focus detector to perform the focus detection in accordance with the detection result of the unbalance state detector.
Japanese Patent Laid-Open No. 2000-165740 discloses a camcorder which is configured so that a charge accumulation time is lengthened in order to prevent the deterioration of the play quality due to the shortness of the charge accumulation time of the image pickup element of the camcorder at the time of a video shooting. Japanese Patent Laid-Open No. 7-298002 discloses an image pickup apparatus which is configured to set the charge accumulation time of the image pickup element to a fixed value based on a blinking cycle of a flicker in order to prevent the deterioration of visibility due to the variation of the luminance in each frame of a video.
However, in the configuration disclosed in Japanese Patent No. 3592147, when an opening of the aperture controlling a light intensity at the time of shooting is equal to or less than an opening of a light shielding layer arranged at the front of the focus detection pixel, the vignetting is generated by the aperture in an incident light beam to the focus detection pixel. When an image signal outputted from the focus detection pixel is disordered by the vignetting, precise focus detection cannot be performed.
In the configuration disclosed in Japanese Examined Patent Publication No. 7-62731, when an opening of the aperture controlling the light intensity at the time of shooting is narrower than each of openings of the first and second focus detectors, the vignetting is generated in the incident light beam to each focus detector and precise focus detection cannot be performed.
In a case where the focus detections disclosed in Japanese Patent No. 3592147 and Japanese Examined Patent Publication No. 7-62731 are performed by a camcorder, when the aperture setting is controlled to be a small aperture side, the vignetting is generated in the focus detection pixel and precise focus detection cannot be performed. Further, in such focus detections, when the charge accumulation time of the image pickup element is set to a fixed value based on a blinking cycle of the flicker in shooting under the flicker illumination, the adjustment of an exposure amount is performed by the aperture setting and a signal gain adjustment. Therefore, when the object is bright, the aperture is easy to be stopped down. As a result, the vignetting is generated in the focus detection pixel and precise focus detection cannot be performed.